The present invention relates to image processing, and in particular to an apparatus and method for image processing for the analysis of histological samples, such as slide sections prepared following the resection of a tumor or other neoplastic lesion.
When removing a cancerous tumor or other type of lesion, it is desirable to remove as little healthy tissue as possible while ensuring that all of the abnormal or cancerous tissue is removed. Metastasis may occur if malignant cells are missed. Thus in order to assess the success of a surgical procedure to remove a lesion, it is the usual practice among hospitals today to inspect the resected tissue following surgery. This inspection is performed by checking to see that healthy tissue is present at all margins of the tissue. Tumor-free margins at the surgical site offers the best chance for the patient to avoid local recurrence of cancer following tumor resection. In 40 to 50% of all cancer cases, however, local recurrence does in fact occur due to inadequate margins.
The current practice for assessing tumor margin relies heavily upon a histological analysis typically performed by a pathologist. Slides are prepared by sectioning the tissue, and these slides are then visually assessed by the pathologist. The pathologist looks for indications of isolated cancerous or abnormal cells at the margins. Using current technology, isolated islands of malignant cells may only be observed via direct microscopic evaluation of ex vivo tissue. Any alternative technique for examining tumor margin, such as using images prepared from the tissue slides, would require that the images be of sufficiently high resolution to resolve individual malignant cells. Imaging techniques such as sonography, digital mammography, and magnetic resonance, for example, do not produce images of sufficient resolution to identify single malignant cells in a tissue sample.
Microscopic evaluation of resected tissue requires the preparation and examination of a number of tissue slides. The slides each contain a cross-section that is cut across the tissue sample. In the simplest case of a biopsy requiring the assessment of surgical margins, the technician or pathologist assistant will usually identify the margins grossly and paint them with an insoluble dye such as India ink. Other cases, such as breast cancer lesions, for example, require a much more complex preparation and examination procedure. Excised breast tumors typically undergo a rigorous protocol including but not limited to an examination of the gross specimen with comparison to radiographic images; touch preparations of all surgical margins; inking of surgical margins with specific colors corresponding to specific margins; serial sectioning of the specimen; and measuring of the distance of the tumor to the closest surgical margin at all sides for each section.
Even with the complex procedure used for post-surgical inspection of breast cancer lesions as described above, it is still impossible to know with absolute certainty that a lesion has been entirely resected. Touch preparations may yield false results. The inking of surgical margins is complicated and mistakes may occur. Perhaps the most significant source of error, however, is that only a relatively small portion of the specimen is being imaged in the sectioning protocol. In a typical sectioning regimen, only 6-12 slides having a thickness of about 5 μm are submitted for histological examination. Those sections are typically cut from the excised tissue at about 3 mm intervals. In some cases, however, as few as one section may be submitted, and conclusions are based upon the presence or absence of the lesion in that section and, if present, its distance from the surgical margin. Even in the case where 6-12 slides are submitted, such slides represent only a small fraction of the total volume of resected tissue. For example, supposing that a tissue sample containing a 2 cm round neoplasm is submitted, a total of up to eight histologic sections may be examined. Using specimens of 5 μm thickness, it may be seen that only about 40 μm of the total 2000 μm-thick neoplasm will be examined by the pathologist, representing only 2% of the total lesion.
Due to the limited number of slides prepared, the accuracy of the sectioning approach to post-surgical evaluation as described above relies upon the lesion being of reasonably regular shape. Many types of lesions, however, are known to take forms that are highly irregular or even discontinuous. Thus if the irregularity occurs in an interval between the regions that are prepared as slides, the pathologist may well miss the fact that a section of the tissue has no healthy margin, indicating that cancerous tissue remains behind at the region of the resection. Although simply increasing the number of slides that are prepared and reviewed by the pathologist would reduce the risk associated with this source of error, that alternative is generally not practical in a clinical setting. To view even a majority of the area of a tumor using a typical specimen size of 15×15×15 mm and using slides of 5 μm thickness, more than 200 slides per procedure must be viewed. This number of slides would occupy a pathologist for an entire working day. Even if enough pathologists were available to review such a large number of slides per procedure at most hospitals, the cost of this type of comprehensive review would be prohibitive.
The inventors hereof have investigated the use of high-resolution microscopic digital photographs as a replacement for the direct examination of tissue sections under the microscope. To be useful for histopathology, however, the digital photographs used would need to have a very high resolution, on the order of 54,000 pixels per inch or higher. At this resolution, a single image of a 15 mm×15 mm area would produce a digital file of a prohibitively large size of approximately 2.8 Gb. Microscope scanners are now available that provide the necessary resolution, some such devices producing images at up to 100,000 dpi. The inventors are unaware, however, of any commercially available software package capable of processing images of such a large size. The task is complicated by the large number of slides that must be examined per specimen in order to ensure that a sufficient percentage of the specimen is examined, and thereby achieve the greatest improvement of histopathology success. The computing resources typically available to most hospital pathology laboratories would, in any event, be insufficient to the task of timely processing the enormous amount of information contained in a series of high-resolution digital photographs as described.
A method of determining the margin of neoplastic lesions in resected tissue that utilizes a greater number of specimen slides for increased accuracy, and that does not require an increased period of pathologist histological review, is thus desired. Furthermore, such a method utilizing high-resolution digital photographs yet requiring no specialized computing equipment would also be desired.